1. Field of the Invention
The present invention relates to an integrated differential pressure sensor and to a manufacturing process thereof.
2. Description of the Related Art
Differential pressure sensors are made using known semiconductor technology. FIG. 1 illustrates by way of example a differential pressure sensor 1 of a piezoresistive type.
In detail, the differential pressure sensor 1 comprises a substrate 2 of semiconductor material (typically silicon), having a cavity 3 dug and accessible from the back of the substrate 2 and a flexible membrane 4 suspended above the cavity 3. Piezoresistive elements 5, connected in a Wheatstone-bridge configuration, are diffused in a surface portion of the flexible membrane 4 and are contacted by metallizations 6, and a passivation layer 7, made of thermal oxide, coats the top surface of the substrate 2. The back of the substrate 2 is bonded to a base layer 8, preferably made of Pyrex™ glass, or alternatively of silicon. The joining between the substrate 2 and the base layer 8 can be, for example, guaranteed by an intermediate layer 9, of a lead-based paste (glass frit). An access opening 10 traverses the base layer 8 and the intermediate layer 9, and reaches the cavity 3.
In use, the top side of the flexible membrane 4 (i.e., the side opposite to the cavity 3) is placed in communication with a first chamber (not shown) containing a fluid at a first pressure, and the cavity 3 is placed in fluid communication with a second chamber (not shown), containing a fluid at a second pressure, through the access opening 10. Consequently, the flexible membrane 4 is deformed as a function of the difference between the first pressure and the second pressure, and said deformation brings about an unbalancing of the Wheatstone bridge formed by the piezoresistive elements 5. Said unbalancing may be detected by appropriate sensing electronics, which derives therefrom the desired differential pressure measurement.
FIG. 2 shows a package 11 of a known type housing the differential pressure sensor 1. In detail, the package 11 is made of thermoplastic material, and has a chamber 12, to a bottom internal surface of which the base layer 8 of the differential pressure sensor 1 is bonded via a layer of adhesive material 13. The chamber 12 is filled with a silicone coating gel 14, and is closed at the top by a metal cover 15, which further delimits a main top surface of the package 11. The silicone coating gel 14 surrounds and coats the differential pressure sensor 1, and acts as a protection against the external environment. The metal cover 15 has a first opening 16, which is placed, in use, in fluid communication with the first chamber. Furthermore, the base of the package 11, in a position corresponding to the access opening 10, has a second opening 17 connected to the access opening 10 and placed, in use, in fluid communication with the second chamber. The electrical connection between the differential pressure sensor 1 and the outside of the package 11 is provided via metal leads 19, which come out of the package 11.
Alternatively (FIG. 3), packages 11 are known comprising a ceramic base 22, bonded to which is the differential pressure sensor 1, and a metal casing 23, which encloses the differential pressure sensor 1 and rests on the ceramic base 22 in contact therewith. The metal casing 23 is open at the top to form a first port 24, which is placed, in use, in fluid communication with the first chamber. Furthermore, the inside of the metal casing 23 is filled with a silicone coating gel 25, which surrounds and coats the differential pressure sensor 1. Through the ceramic base 22, in a position corresponding to the access opening 10, a passage 26 is provided, through which a second port 27 is placed in communication with the access opening 10. Furthermore, in use, the second port 27 is placed in fluid communication with the second chamber. Electrical connection between the differential pressure sensor and the outside of the package 11 is provided via metal leads 28, which come out of the ceramic base 22 through further passages 29 provided in the ceramic base 22.
The pressure sensor described, though enabling a differential pressure measurement to be carried out, has, however, rather large dimensions, principally due to the need to perform a digging from the back of the substrate 2. The manufacturing process, for similar reasons, is rather complex and costly, principally due to the need to perform the digging from the back (generally via a TMAH etching) and the bonding between the substrate 2 and the base layer 8. Clearly, said disadvantages are particularly evident in applications wherein features such as economy and simplicity of production are constraining design characteristics.